This application claims the priority of German Application No. 197 55 815.1, filed Dec. 16, 1997, the disclosure(s) of which is (are) expressly incorporated by reference herein.
The present invention relates to a process for the water vapor reforming of a hydrocarbon in a heated environment. The present invention also relates to a reforming system which can be operated by this process. The present invention is particularly directed to a process for operating a mobile system for the water vapor reforming of methanol in a fuel-cell-operated motor vehicle for providing the hydrogen required for the fuel cells, and a system which can be operated in this manner, as well as to an operating process of a fuel cell system. For reasons of simplicity, the term "hydrocarbon", in addition to actual hydrocarbons, will also include its derivatives, such as methanol.
Japanese Published Patent Application JP 4-338101 (A) discloses a process for starting a fuel cell system fed with hydrogen by a reforming reactor. In this case, the anode exhaust gas of the fuel cells is fed to a hydrogen storage device. During starting, hydrogen from the hydrogen storage device is fed to a catalytic burner, which is in thermal contact with the reforming reaction space, and is burned therein.
In the case of a hydrogen generating system described in Japanese Published Patent Application JP 8-231202 (A), the hydrogen is obtained by the exothermal partial oxidation of methanol. By means of the heat generated thereby, a heat-conducting feeding conduit is heated to evaporate methanol fed thereto before reaching the reaction space.
U.S. Pat. No. 5,248,566 describes a fuel cell system wherein the fuel cells are fed by hydrogen on the anode side. The hydrogen is generated by a partial oxidation reforming reactor. The anode exhaust gas of the fuel cells is burned in an afterburner together with air fed thereto. The resulting generated heat is used to heat the interior of a motor vehicle which is equipped with the fuel cell system.
With a selectively hydrogen-permeable membrane, a membrane module is used to separate hydrogen generated when the system is warmed up as the result of the reforming reaction of the other constituents of the formed reformate gas. In addition to alternative approaches, such as CO conversion to carbon dioxide by CO oxidation, or the CO shift reaction, this represents a method for obtaining a product gas which consists essentially of hydrogen wherein CO concentration in the product gas does not exceed a defined low threshold value. This is important, for example, when the product gas is used as the anode gas of a fuel cell system, since there the carbon monoxide has the effect of a catalyst poison. The membrane module can be connected as a separate unit behind the reforming reactor or can be integrated in the reforming reactor. Systems of the latter type are described, for example, in German Patent Document DE 44 23 587 C2.
It is known that the water vapor reforming reaction for reforming a hydrocarbon or hydrocarbon derivative, such as methanol, takes place endothermally and at a reaction temperature which is higher than room temperature. During a cold start of the system, the water vapor reforming reaction may not immediately provide hydrogen. The system components must first be brought to a corresponding operating temperature. However, particularly when these systems are used in motor vehicles, it is desirable to have driving power for the fuel cells available as soon as possible after triggering the starting operation of the vehicle and thus also of the reforming system. This requires that the reforming system be capable of providing hydrogen as fast as possible at expenditures which are as low as possible. Various special measures for the cold start of reforming systems have been suggested for this purpose.
Thus, it is known from French Patent Documents FR 1.417.757 and FR 1.417.758, which are cited in the above-mentioned German Patent Document DE 44 23 587 C2, to introduce, during a cold start of a system for water vapor reforming of methanol, first a mixture of methanol and an oxidant into the reforming reactor in order to carry out a corresponding combustion reaction there and thus heat the reactor. Thereafter, the oxidant feed is terminated. Instead, a methanol/water vapor mixture to be reformed is fed to the reactor and the water vapor reforming reaction is started. In the system of French Patent Document FR 1.417.757, a heating space is in thermal contact with the reforming reaction space. The residual gas from the reaction space which is not diffused through a separating membrane is non-catalytically burned with oxygen in the heating space.
Special cold starting measures were also suggested for systems for water vapor reforming of a hydrocarbon without the use of a hydrogen separation stage. U.S. Pat. Nos. 4,820,594 and 5,110,559 describe systems for water vapor reforming of a hydrocarbon wherein a burner is integrated in the reforming reactor. The burner is in thermal contact with the reaction space of the reactor by way of a heat-conducting partition. During cold start, a combustible mixture is burned in this burner in an open flame. In U.S. Pat. No. 5,110,559, the mixture originates from the reforming reactor itself, in which case the combustible hydrocarbon to be reformed has already been fed to the reaction space during the cold start. The hot combustion exhaust gases of the burner integrated in the reactor are transmitted into a CO shift converter in order to heat this CO shift converter and in this manner bring the system more rapidly to the operating temperature.